This invention relates to pneumatic ejectors for use in sorting apparatus in which material to be sorted is directed in a product stream following an aerial path, and rejected material is removed from the stream by air pulses from such ejectors. Particularly, it relates to valve devices for use in the ejectors to control the delivery of air from a pressurised source to the ejector nozzle directed at a particular section of the product stream. Although described with reference to sorting apparatus, it will be recognised that the invention will have application in other technical fields where the delivery of a gas under pressure calls for careful and accurate control.
Pneumatic ejectors are now commonly used in sorting apparatus. Essentially, each will comprise a nozzle connected via a valve device to a source of pressurised air. An operating mechanism, normally comprising an electric circuit, acts to open the valve device selectively in response to a control signal to deliver a pulse of pressurised air to, and thus from the respective nozzle. As the operating speeds for sorting apparatus increase, so must the action and response of valve devices in the ejectors.
Many current sorting apparatus using pneumatic ejectors grade particulate material according to its ability to reflect light. Typical such apparatus are described in U.S. Pat. Nos. 4,203,522; 4,513,868; and 4,699,273, the disclosures whereof are incorporated herein by reference. In apparatus disclosed in the '522 Patent detectors are responsive to light reflected from the particles and generate signals indicative of different qualities of the product. These signals are compared and analysed, to generate a comparison signal which can activate an ejector to remove the relevant particle from the product stream. An ejector in these apparatus will normally use an electrically actuated valve such as a moving iron disc or plate valve to discharge pulses of pressurised air selectively to remove particles from its respective target area.
Air valves of various kinds have been developed based on piezoelectric actuators. A typical piezoelectric actuator has one or more lamina of piezoelectric ceramic bonded together, or to an additional non-piezoelectric laminar such as a metal plate, to form a bimorph or multimorph structure. An electrical voltage is applied to the piezoelectric ceramic laminar through a layer of metallisation across the piezoelectric laminar structure or diaphragm. The electrical voltage causes the piezoelectric ceramic to expand or contract depending on the polarity. When one expanding or contracting laminar is bonded to another which expands in the opposite direction, or to a laminar that does not change dimensions, the resulting structure will bend when an electrical signal is applied. Thus in a valve device, the actuator may be positioned to cover a valve orifice, or hold another object over the valve orifice, in one electrical state; and to move away from the orifice and open the valve in another electrical state.
Piezoelectric actuated valves are disclosed in U.S. Pat. Nos. 5,079,472 and 5,343,894, and Canadian Patent Specification No. 2,093,257, to which reference is directed.